A dual fuel engine can typically operate in two modes. In a strictly liquid fuel mode a liquid fuel, such as diesel fuel, is injected directly into an engine cylinder or a precombustion chamber as the sole source of energy during combustion. In a dual fuel mode a gaseous fuel, such as natural gas, is mixed with air in an intake port of a cylinder and a small amount or pilot amount of diesel fuel is injected into the cylinder or the precombustion chamber in order to ignite the mixture of air and gaseous fuel.
A variety of techniques have been used in the past to determine the amount of gaseous fuel to deliver to an engine and the amount of liquid fuel to deliver to the engine during the dual fuel operating mode. For example, some techniques operate by allowing the fuel system governor to adjust the amount of diesel fuel delivered based upon changes in engine speed which occur after the amount of gaseous fuel delivered is changed. These techniques can result in undesired fluctuations in engine speed due to changes in the total fuel energy delivered to the engine.
Another problem encountered in dual fuel engines is that the energy content of gaseous fuel can vary. A fuel delivery method which takes into account the varying energy content of the gaseous fuel would therefore be advantageous.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.